Hi,
1. Could you make a screenshot of the problem ?
2. There is no way to do that at the moment.
3. There is no login plugin in hikashop, just a user synchronization plugin so that data is synchronized. If you want you can create a new joomla menu of type HikaShop and select user cpanel layout. Upon clicking on the menu, your users will be able to manage their orders and their addresses. And in the orders, they will be able to download their files.
4. In the tab display of the config, you have the option "Continue shopping button's URL" where you can enter a URL. If you do so, a continue button will be displayed on the checkout and users will be able to click on it and will be redirected to that URL.